1. Field of the Invention
This invention relates to devices and methods for using a magnetic field to guide a surgical implant, and more specifically to devices and methods for using the near field and transition field of a repositionable magnet to move, guide, and/or steer a magnetic seed, catheter or other magnetic delivery vehicle (MDV) for therapeutic or surgical purposes.
2. Description of Related Art
In the field of surgery, there exists a need to control the orientation, forces, and/or motion of internally implanted devices. One method that has been used to control such implanted devices is the application of a magnetic field from an external magnet. In this method, the magnetic field acts upon the implanted device, which itself comprises magnetic material, which may be in the form of a permanent magnet. In accordance with prior art practice, a physician surgically implants the device comprising magnetic material and then guides the position of the implanted device by moving an external permanent magnet and observing the resultant movement directly with an X-ray fluoroscope. Examples of the prior art may be found in a review article by Gillies et al., "Magnetic Manipulation Instrumentation for Medical Physics Research," Rev. Sci. Instrum. 65, 533 (1994), and references cited therein. See also McNeil et al., "Functional Design Features and Initial Performance Characteristics of a Magnetic-Implant Guidance System for Stereotactic Neurosurgery," IEEE Trans. Biomed Engrg., 42, 793 (1995); Tillander, "Magnetic Guidance of a Catheter with Articulated Steel Tip," Acta Radiologa 35. 62 (1951); Frei et al, "The POD (Para-Operational Device) and its Applications," Med. Res. Eng. 5,11 (1966); U.S. Pat. No. 3,358,676 to Frei et al., issued Dec. 19, 1967, entitled "Magnetic Propulsion of Diagnostic or Therapeutic Elements Through the Body Ducts of Animal or Human Patients"; Hilal et al., "Magnetically Guided Devices for Vascular Exploration and Treatment," Radiology 113, 529 (1974); Yodh, et al., "A New Magnet System for Intravascular Navigation," Med. & Biol. Engrg., 6, 143 (1968); Montgomery et al., "Superconducting Magnet System for Intravascular Navigation," Jour. Appl. Phys. 40, 2129 (1969); U.S. Pat. No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled "Magnetically Guidable Catheter-Tip and Method"; and U.S. Pat. No. 3,794,041 to Frei et al., issued Feb. 26, 1974, entitled "Gastrointestinal Catheter." The full content of each of the cited documents are herein incorporated by reference in their entirety.
Unfortunately, the above-described technique requires the physician to react to the movement of the implanted device after the fact. There is no precise correlation of the imaging system with the medical magnetic manipulation, and no way to apply fields and/or force gradients precisely in needed directions. With hand-held magnets, the only feedback the surgeon could have was his observation of motion of a magnetic implant by x-ray or ultrasonic imaging in response to his movement of the magnet. The field producing magnet, so guided without direct visual display of the field lines, is controlled by the operator's estimate of the field direction and magnitude at a particular location of the implant. Since many combinations are possible, this essentially "blind operation" is bound to result in a somewhat random position and angulation as related to the needed field line direction and magnitude to provide guidance and/or pulling force. In difficult interference situations, it is difficult without such imaging guidance to provide even a reasonable guess as to a correct direction for the magnet axis to obtain field alignment with the intended path. The large electromagnet of Yodh et al. is one attempt to minimize the "blindness" of the approach just described, but the Yodh et al. approach still relies on operator judgment and vision, and is subject to such error. While multiple coil arrangements such as the magnetic stereotaxis system (MSS) described in McNeil et al. can be used to provide such guidance, it is difficult in such systems to provide a combined guiding force and force-applying field gradient in the same desired direction.
Clearly, both operation time and risk to a patient could be reduced if an apparatus and method were available to more accurately and rapidly guide or move a magnetic surgical implant. This device and method can either provide feedback to the physician guiding the implant so that the physician could predict the movement of the implanted device rather than react to it after the fact, or it can be used more automatically with computer-controlled motion along a physician-selected planned path. It would also be advantageous if simpler hardware and software could be used to locate the external magnet and provide more effective field solutions. The moveable magnet location should take into account an exclusion volume around the patient in which the magnet may not be located. In the case of neurosurgery, for example, the magnet cannot be located too closely to the patient's head, nor in the path of imaging X-rays.